Dip Class is a piece of gamer slang that emerged into the /tg/ world as a result of Dungeons & Dragons 3rd Edition, although seeds of it had been sown in its precursor, Advanced Dungeons & Dragons. Born out of the conflux of multiclassing and Linear Warriors, Quadratic Wizards, the term "dip class" signifies a class that you "dip into" (only take a small number of levels from) in order to gain increased potency when combined with the features of your other, primary class. Generally speaking, the dip class is usually considered too weak to be worthwhile to follow extensively, but it has traits that mesh well with another, stronger class. Even if a class is worth following to the end, strong class features gained early on that either 1: don't scale or 2: have scaling that is not dependent upon class level can make a class worth dipping to use these features on a different class.
The original form of this concept came about in Advanced Dungeons & Dragons, when humans had the unique rule of "dual-classing". It was extremely common - so much so that AD&D-based video-games like Baldur's Gate often have guides advising players to do so - to have player start out as a fighter, for those precious low-level hit points, THAC0 and weapon slots, and then dual-class to Wizard, which would allow you to access its superior end-game abilities.
3e invented this concept, and Pathfinder arguably perfected it, what with the latter's enormous roster of classes and their wide swing of usefulness.
The concept almost died in Dungeons & Dragons 4th Edition, due to the AEDU System and how multiclassing & hybrid classing worked; it did still happen, but it wasn't quite as prevalent (or as cheesy) as it had been in the last edition.
Dungeons & Dragons 5th Edition brought it back, but at least added some depth to it; most now recommend dipping into a secondary class at least enough to get its first subclass features. Also, caster/caster and martial-primary/caster-dip dip-classing has become more prevalent; the paladin with a few levels in sorcerer or warlock is widely considered one of the best class combos in 5e.
Dipping is often used to qualify for a Prestige Class, especially one that advances the dipped class. The requirements for Prestige Classes generally expect dips for entry. Prestige classes may also be dipped, both for the same reason base classes are, and because they lose casting advancement at higher levels. Using a prestige class that is strictly better than the base class (i.e., pretty much any prestige class with full casting advancement and actual class features), generally does not make the base class a dip, even if entered with a minimum of levels via early entry cheese.
Diping classes is a good way to show how retarded Favored Class is. Dipping 1 level in every class doesn't incur penalties, but a Barbarian 6/Ranger 3 is crippled.
Classes typically dipped instead of taken fully include
- Fighter: Two levels gives all proficiencies and two bonus feats.
- Monk: Two levels gives a random list of proficiencies, wisdom to armor class, evasion and two feats.
- Paladin: Two levels give all proficiencies, immunity to fear, and charisma to saves require no scaling, which are all are welcome on any charisma based class. Having a single level is sufficient to activate magic items that use Paladin spells, and three gives another immunity. Advancing just gives you more smites and some casting that keys off the wrong stats.
- Duskblade: In addition to proficiencies, their main ability isn't restricted to Duskblade spells and using Duskblade in an Eldritch Knight build lets them use a far higher rate of casting with it. Despite being dipable, Duskblade is quite playable on its own.
- Hexblade: Is quite similiar to Paladin in this regard. Mettle and Resitance both have no scaling, and while their curse is dependent upon level, it's a free action to attempt and still scales slightly from charisma alone.
- Spellslinger Wizard: In exchange for gimped casting, this archetype gives the ability to cast spells through guns. This ability is not restricted to Wizard spells, and it always better to take only a single level in this archetype plus 19 in any other full caster than using it in any other way.
- Swashbuckler: Both the 3.5 and Pathfinder versions give everything of value in the first few levels and have nothing worth staying for.
- Warlock: While a playable class in its own right, many least evocations give all day buffs that can be quite useful. For anyone with a source of sneak attack or other bonus damage, Eldritch Blast can be a potent weapon on its own.